1. Field of the Invention
The present invention relates to a process for producing aluminum nitride. Specifically, it relates to a process for producing aluminum nitride in which a nitriding reaction is developed after occluding nitrogen in an aluminum powder.
2. Description of the Related Art
Aluminum nitride (i.e., AlN) is good in terms of characteristics, such as heat resistance, thermal conductivity and electric conductivity, its applicable uses have been expanding. As for the applicable uses of aluminum nitride, it is possible to name containers for high-temperature applications, IC substrates, and so forth.
Industrial aluminum nitride is artificial ceramic which does not exist in nature. It is produced mainly by using two methods, the carbon reduction nitriding method of alumina and the direct nitriding method of aluminum.
(a) Carbon Reduction Nitriding Method of Alumina(i.e., Al3O2+3C+N2=2AlN+3CO)
The carbon reduction nitriding method of alumina is a production process in which alumina having a high purity is reduced at a high temperature (e.g., from 1,700 to 1,900° C.) by graphite and a nitrogen gas and simultaneously aluminum formed by reducing is nitrided by nitrogen, thereby producing aluminum nitride.
However, it takes a long time for the carbon reduction nitriding method to complete the reaction. Accordingly, the production cost goes up so that a problem arises in that the resulting aluminum nitride is more expensive than the other ceramics, for example, silicon carbide (i.e., SiC), alumina, and so on.
(b) Direct Nitriding Method of Aluminum(i.e., Al+N2=2AlN)
The direct nitriding method of aluminum has been used since Briegleb et al. succeeded first in the synthesis in 1862. Since the reaction is an exothermic reaction, the direct nitriding method has an advantage in that it is possible to produce aluminum nitride with ease by simply putting pure aluminum in a nitrogen stream.
However, in the direct nitriding method of aluminum, the supply of nitrogen into aluminum is shut off by a nitrided film when the surface of aluminum is covered with the nitrided film. When the supply of nitrogen is shut off, the nitriding reaction of aluminum is terminated. Accordingly, the direct nitriding method of aluminum has a disadvantage in that it is not possible to obtain aluminum nitride of 100% purity. Consequently, the nitriding is carried out industrially while aluminum is heated to a temperature of from 1,000 to 2,000° C.
Moreover, in the direct nitriding method of aluminum, nitriding and pulverizing the resulting aluminum nitride are carried out repeatedly in order to raise the reaction yield, or an additional treatment, such as adding AlF3 or AlN, is carried out in order to complete the reaction. Accordingly, the direct nitriding method of aluminum requires a step of pulverizing the bulks of aluminum nitride. As a result, the production cost goes up. Hence, the direct nitriding method of aluminum has a problem in that the resulting aluminum nitride is highly expensive.
In addition, the direct nitriding method of aluminum has a problem in that aluminum which is not nitrided remains in the resulting aluminum nitride in a trace amount. When the aluminum resides in the aluminum nitride, the electric characteristics of the aluminum nitride degrade.
In general, aluminum has an oxide film on the surface. Due to the oxide film, the nitriding of aluminum is carried out in a liquid phase region within a temperature range of from 900 to 1,400° C. On the other hand, it is possible to nitride aluminum in a solid phase region within a temperature range of from 520 to 650° C., however, there is a problem in that the nitriding reaction is terminated in the middle so that aluminum nitride of virtually 100% purity cannot be obtained. The problem results from the fact that the nitriding reaction is terminated by an aluminum nitride film which is formed when aluminum is nitrided by 95 to 98.5%.